[HLSL] Implement default constant buffer `$Globals`

[hekota]

All non-resource variable declarations in the global scope are implicitly added to $Global constant buffer.

In Sema we need to recognize these declarations and assign them hlsl_constant address space. At the end of Sema we will probably need to generate an implicit HLSLBufferDecl for the $Globals constant buffer and create its layout struct. Then we need to add support for it in codegen, making sure the code is generated in a similar way as other cbuffers.